An Israeli man tries a gas mask at a distribution centre in Or-Yehuda, south of Tel Aviv on February 28, 2010. Israel began distributing new gas masks for use in a possible chemical or biological attack, the army said.

Bashar al-Assad's regime in Syria has armed "dozens of 500-pound bombs" with Sarin nerve gas, according to a report based on satellite imagery seen by the Pentagon at the end of last year.

The details of the briefing were released to the New York Times, explaining a sudden warning issued by President Barack Obama to the Syrian regime at the beginning of December.

Israel says it notified the Pentagon that Syria was preparing a chemical — believed to be sarin gas — and loading it into dozens of 500-pound bombs.

The warning came at the end of November and brought together the US, Arab states, Russia and China to deal with Syria's deadly civil war, The New York Times wrote.

“Without getting too deep into matters of intelligence, I’m unaware of any information that would suggest that the Syrians are planning the imminent use of chemical weapons or deployment of chemical weapons. Let me be very clear, that this government would view that kind of action as a red line."

Little called the violence perpetrated against Syrians by its own government was "unacceptable."

He added:

"It’s depraved. And it’s time for the Assad regime to go. But in the meantime, they have a serious responsibility to maintain security over their chemical weapons stockpiles and not to use them."

Haaretz, meantime, reported that Russia was making progress in its effort to safeguard Syria's chemical weapons — part of an agreement between Secretary of State Hillary Clinton and her Russian counterpart Sergei Lavrov.

The pair agreed last month that Washington would not support foreign military intervention in Syria as long as Moscow contained the Assad regime's use of chemical weapons.

Amid diplomatic efforts to find a solution to the Syrian crisis, Russia has moved to safeguard Syrian chemical weapons directly with Assad, Haaretz wrote, monitoring how they are stockpiled.

Chemical weapons were no longer at grave risk of falling into rebel hands and Syria had promised not use them.